1. Field of the Invention
The present invention is generally related to a structure and process for forming dense multi-layer thin film structures and, more particularly, to a method of making individual layers of thin film wiring which can be pre-inspected and then laminated one on top of the next to form a three dimensional wiring matrix.
2. Description of the Prior Art
The fabrication of dense multi-layer thin film interconnect structures for high-density integrated circuits typically involves the sequential build-up of metal-patterned dielectric layers on silicon wafers or ceramic substrates. A first layer is formed on top of the substrate, then a second layer on top of the first and so forth in series. Among the various dielectric/insulator materials in thin film structures, sputtered (or PECVD) quartz, silicon nitride, and high-temperature stable polymers, especially polyimides, are most commonly used in conjunction with high-conductivity metallurgy such as aluminum/copper (Al/Cu), gold (Au), and/or copper.
The sequential build-up approach suffers from several limitations. First of all, every time a new layer is fabricated, the previously built layers are exposed to the entire process excursions, including thermal and chemical solvents, as well as mechanical and other stress-related operations. Secondly, since the electrical performance and long-term reliability of the resulting structure can only be determined after the conclusion of the entire fabrication process, the finished structure may have to be discarded if the performance does not meet the required specifications. This results in high cost of production and has other obvious limitations in terms of cycle time and throughput. In addition, when multilayer thin film is built directly on the substrate, it is not possible to build small sections of high density wiring, since the entire substrate must be coated and processed with the thin film.
When using polyimide dielectric or other high-temperature polymers, an alternate approach to fabricating thin film interconnect structures is based on the assembly of individual electrically inspectable metallized thin films (layers) which are laminated at high temperature such that metal-to-metal and polymer-to-polymer bonding can be achieved. This method eliminates some of the limitations of sequential processing, as each metallized dielectric layer is fabricated as a single unit which can be fully inspected for the desired electrical characteristics, then multiple layers are stacked and laminated under heat and pressure. However, although the method of transferring individually inspectable layers is potentially superior to the sequential build-up of layers, it has a fundamental problem with regard to the dimensional stability of the structure during both individual layer build and during the joining of the individual layers to form the composite structure. This is due to the fact that the thin polyimide films are generally fragile and are subject to deformation under thermal or solvent-related stress conditions. This can result in pattern misalignment and distortion during layer fabrication or in the process of lamination, and also when the composite structure undergoes accelerated reliability tests involving temperature and humidity excursions.